1. Field of the Invention
This invention relates to an electrophotographic technique of forming an image by modulating ion current through an electric field formed on a screen photosensitive member.
2. Description of Prior Arts
From U.S. Pat. No. 3,582,206, U.S. Pat. No. 3,645,614 and others, there has already been known an electrophotographic method, by which a primary latent image following an original image is formed on a screen photosensitive member having a multitude of fine openings (such photosensitive member will hereinafter be called simply "screen"), and this latent image is used for modulating corona ion current to form a secondary latent image on a chargeable surface, thereby reproducing the original image.
The screen photosensitive member as disclosed in British Patent specification No. 1,480,841 and U.S. Patent Application Ser. No. 771,309 and for use in such electrophotographic method as disclosed in the abovementioned U.S. Patents is of such a construction that it has an electrically conductive substrate, a photoconductive layer provided on the electrically conductive substrate, a surface insulative layer on this photoconductive layer, and a further electrically conductive member at the side of a modulating corona source.
The effects to be derived from the abovementioned laminar structure of the screen are as follows:
(1) The latent imge charge as formed on the screen is less attenuable with lapse of time and passage of ion, because the primary latent image is formed on the insulative layer; and
(2) Owing to the presence of the electrically conductive member at the side of the modulating corona source, excessive or unnecessary modulating corona flows out of the electrically conductive member at the time of modulation, and such excessive corona ion at the modulation does in no way have an adverse effect on to the primary latent image.
On account of such effect derived from the presence of the surface insulative layer and electrically conductive member at the side of the modulation ion source, it is possible to perform retention copying which modulates ion current over many repeated times with one and the same primary latent image.
In the following, detailed explanations will be given in reference to FIGS. 1 to 5 of the accompanying drawing as to this known screen disclosed in the abovementioned British patent specification and U.S. Pat. application as well as examples of the latent image formation using such conventional screen.
It should be noted that the materials used for the screen will be omitted from the explanations.
FIG. 1 is a schematic diagram of a partially enlarged cross-section of the screen disclosed in the abovementioned prior patent and application. In the illustration, the screen 1 is of such a construction that the photoconductive layer 3 and the surface insulative layer 4 are laminated on the electrically conductive member 2 having a multitude of fine openings. In the following explanations, this electrically conductive member 2 has such a characteristic that permits electrons to be injected into the photoconductive layer even at a dark portion of the image. More concretely, the photoconductive layer 3 is composed of cadmium sulfide (CdS), zinc oxide (ZnO), and other like semiconductors with electrons as the principal carrier.
FIGS. 2 to 5 explain the primary and secondary latent image forming processes, in which FIG. 2 illustrates the primary voltage application step, FIG. 3 the secondary voltage application step, FIG. 4 the overall irradiation step, and FIG. 5 the electrostatic latent image forming step, wherein ion current is modulated by the primary electrostatic latent image formed on the screen by the foregoing steps.
FIG. 2 shows the primary voltage application step, wherein the screen 1 is uniformly charged in the positive (+) polarity by a corona discharger as the voltage application means. In the illustration, a reference numeral 5 designates the corona discharger. By the above-mentioned electric charging, the positive charge is accumulated on the surface of the insulative layer 4. On account of this charge, a negative charge layer which is in the opposite polarity of the above-mentioned charge is formed in the vicinity of the insulative layer 4 in the photoconductive layer 3. Incidentally, when an interface between the photoconductive layer 3 and the electrically conductive member 2 as well as the photoconductive layer per se are of such a property that majority carriers are injected thereinto, but minority carrier are not, and the screen as a whole possesses a rectification property, it is possible to form the charge layer in the vicinity of the insulative layer 4 in the photoconductive layer 3 even at the dark portion by the carrier injection as mentioned above. In the screen having no rectification property, or no capability of forming the abovementioned charge layer by application of the primary voltage, satisfactory result will be obtained with the charging method as described in U.S. Pat. No. 2,955,938, wherein the insulative layer is charged at a bright place.
FIG. 3 illustrates a result of simultaneously effecting both image irradiation and secondary voltage application step on the screen 1 which has been subjected to the primary voltage application step. In the drawing, a number 6 refers to an image original, wherein the side D denotes a dark portion, the side L a bright portion. Arrows 7 indicate light from a light source (not shown). A numeral 8 refers to a corona discharger for the secondary voltage application. In the illustrated case, the screen is charged in the opposite polarity by the corona discharge from a corona wire 8 applied with a d.c. voltage in the negative polarity so that the surface potential of the insulative layer 4 may be in the negative polarity.
When the surface potential of the insulative layer 4 takes the negative polarity as mentioned above, the substance in the photoconductive layer 3 at the bright portion L becomes electrically conductive due to the image irradiation with the consequence that the surface potential of the insulative layer 4 assumes the negative polarity. On the other hand, however, the surface charger of the insulative member 4 at the dark side D therof remains in the positive polarity because of the negative charge layer existing in the photoconductive layer 3 at the side of the insulative layer 4.
Considering, here, the polarity changing speed in the potential on the insulative layer 4 of the screen 1 in the abovementioned step, the portion where the insulative layer 4 faces the corona discharger 8 (the front surface side) changes the most quickly, while the lateral surface part and its vicinity which surround this facing portion and constitutes the opening of the screen changes after this front surface part. Accordingly, at the image irradiating section, the potential of the screen, at its side where the electrically conductive member 2 is exposed (the rear surface side), corresponds to the potential on the electrically conductive member 2, and the potential gradually increases from this rear surface side to the front surface side.
FIG. 4 shows the result of effecting a uniform overall light exposure as the overall irradiation step, to the screen 1 which has been subjected to the image irradiation and the secondary voltage application step. In the drawing, arrows 9 denote a uniform light beam from a light source (not shown). By this overall irradiation, the potential at the dark side D of the screen 1 changes to a potential which is proportional to the charge quantity on the surface of the insulating layer 4.
As the result, there exist a relationship to be represented by the following equation (1) among a latent image contrast Vc at the bright and dark portions of the screen 1, a surface potential V.sub.1 due to the primary voltage application step, and a surface potential V.sub.2 due to the secondary voltage application step. EQU Vc=[Ci/(Ci+Cp)](V.sub.1 -V.sub.2) (1)
(where: Ci denotes an electrostatic capacitance of the surface insulative layer 4; and Cp and electrostatic capacitance of the photoconductive layer).
In order to increase the electrostatic contrast of the primary latent image in the above equation (1), there may be contemplated the following:
(a) a method, in which the surface potential V.sub.1 is elevated by increasing the primary voltage, in accordance with which the latent image potential at the dark portion is elevated; and PA1 (b) a method, in which the secondary voltage is made as low as possible, in accordance with which the latent image potential at the bright portion is lowered. PA1 (i) if the surface potential V.sub.L of the screen corresponding to the bright portion of the image is set at the zero potential which is higher than the surface potential Vcrit., the curve (dot-and-dash line) follows V.sub.1 -V.sub.2 -V.sub.D, V.sub.L, and the potential difference becomes Vc; PA1 (ii) if the surface potential V.sub.L of the screen corresponding to the bright portion of the image is set at the same value as the surface potential Vcrit., the curve (solid line) follows V.sub.1 -V.sub.2 '-V.sub.D ', V.sub.L ', and the potential differences becomes Vc'; and PA1 (iii) if the surface potential V.sub.L of the screen corresponding to the bright portion of the image is set at a value lower than the surface potential Vcrit by performing intense application of the secondary voltage in the opposite polarity as mentioned with respect to FIG. 6, the curve (dash line) follows V.sub.1 -V.sub.2 "-V.sub.D ", V.sub.L ". PA1 (a) uniform changing of the surface insulative layer in a particular polarity; PA1 (b) irradiation of dark and bright patterns of an image onto the photosensitive member; and PA1 (c) uniform charging of the photosensitive member in a polarity opposite to the charge polarity in the step (a), this step being carried out simultaneously with, or subsequent to, the step (b);
In the above method (a), there is a limit to the increase in the primary voltage, since the method is operated generally in an almost critical state with the consequence that excessive electric charge would cause spark discharge or pin holes due to dielectric breakdown to apprehensively impair the screen. On the other hand, the electrostatic contrast can be increased by the method (b), that is in the conventional surface potential curve (V.sub.1 -V.sub.2 -V.sub.D, V.sub.L) as shown in FIG. 6 with a solid line, the negative voltage at the time of the secondary voltage application is raised to lower V.sub.2 to V'.sub.2 in the surface potential curve after the secondary voltage application as shown by a dash line, whereby the electrostatic contrast Vc (.vertline.V.sub.D -V.sub.L .vertline.) to be finally obtained is considerably increases to Vc' (.vertline.Vc'-V.sub.L '.vertline.).
FIG. 5 shows a state, wherein the ion current is modulated by the primary latent image to form the secondary latent image on a recording medium. In the drawing, a numeral 10 refers to a corona wire, 11 an opposite electrode member, 12 a chargeable surface such as a reproduction paper, on the surface of which the secondary latent image is formed. Numerals 13, 14 denote power source sections, by which an electric field is formed in the flowing direction of the corona ion between the corona wire and the reproduction paper 12. The reproduction paper 12 is disposed closer to the side where the insulative member 4 of the screen 1 faces, and the flow of ion is applied to the reproduction paper 12 from the corona wire 10 disposed with the screen 1 interposed between the wire and the paper. In this instance, there acts on the screen 1 an electric field due to the primary latent image; that is, at the bright portion of the image, the field acts to block the negative flow of ion as shown in solid lines .alpha. acts on the screen, while at the dark portion of the image, the field accelerates the flow of ion as shown in solid lines .alpha.. By these electric fields, the secondary latent image is formed on the reproduction paper 12 in the form of the positive image of the original.
Here, it is assumed that the critical value of the field .alpha. to block passage of the negative corona ion at the screen openings at the time of the modulation is given as ".alpha. critical" (hereinafter abbreviated as ".alpha. crit."), this blocking field .alpha. crit. is determined by the diameter and depth of the opening, and, further, by a potential difference between the electrically conductive member 2 and the opposite electrode member 11, whereby the field to be formed by the potential difference V between the front and rear surfaces of the screen becomes a yardstick for it. Moreover, if it is assumed that the potential difference V between the front and rear surfaces of the screen, when the critical field .alpha. crit. is formed by use of the screen 1, is "V critical" (hereinafter abbreviated as "Vcrit."), the surface potential of the screen, when the field .alpha. crit. is formed, becomes inevitably Vcrit., because the electrically conductive member 2 at the rear surface of the screen 1 is exposed outside, hence its surface potential is zero. Accordingly, when the screen 1 is used, passage of the corona ion in a particular polarity through the screen openings is entirely blocked as soon as the surface potential of the screen becomes below Vcrit.
Referring now to FIG. 7, explanations will be given as to the relationship between the electrostatic contrast Vc in the primary latent image using the abovementioned screen 1 and the electrostatic contrast virtually acting in the secondary latent image forming step.
Assume that the critical blocking field .alpha. crit. corresponds to the surface potential Vcrit. of the screen 1:
In the abovementioned three situations (i), (ii), and (iii), the highest primary latent image contrast is obtained when the relationship of Vc&lt;Vc'&lt;Vc" is established from FIG. 7 and the above equation (1), i.e., the case of above (iii). With the surface potential being below Vcrit., the corona ion does not pass through the screen openings, hence no contribution to the secondary latent image formation. As the result of this, the electrostatic contrast becomes substantially V.sub.D "-Vcrit. Accordingly, of the primary latent images, those having the surface potential in a range of from Vcrit. to V.sub.L " are not turned to the secondary latent image. Particularly, the image on the bright portion thereof is not reproduced, hence no reproduced copy, in which the original image has been faithfully reproduced, cannot be obtained. In contrast to this, the above case (i) is lower than the case (ii) in its primary electrostatic latent image contrast. When it cannot be expected that the primary voltage is made higher than V.sub.1, the primary electrostatic latent image contrast becomes the maximum by setting of the case (ii). In other words, the electrostatic contrast of the primary latent image to faithfully reproduce the original image in the abovementioned steps by the use of the screen 1 becomes the maximum when the surface potential at the bright portion of the image is set at Vcrit. as is the case with above (ii).
In the conventional screen, the critical blocking field .alpha. crit. is set by the potential of the primary latent image at the bright portion of the image, and the method as in the above case (iii) in FIG. 7 could not be used, because faithful reproduction of the original image could not be attained.